Consider changing enums to struct(u16)

[jcjones]

J.C.'s thought: we use enum all over, but I'm tempted to make this

#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct CollectionJobExtensionType(u16);

so the boilerplate becomes more compiler-generated.

Originally posted by @jcjones in #4693 (comment)

Give an example of how it goes. Consider documenting the preference either way in the DEVELOPMENT.md